1. Field of the Invention
This invention relates to refrigeration devices and methods that employ a supercritical fluid in a vapor-compression thermodynamic cycle, and more particularly to a small-scale apparatus needed to operate such a cycle. Typical applications include cooling of electrical, electronic, optical and portable devices.
2. Background
Small-scale refrigerators, also referred to as microrefrigerators, are under development for the purpose of cooling such devices as computers, servers, telecommunications switchgear and numerous other types of electronic equipment, as well as portable coolers, medical equipment and many more items that are generally compact in design, if not portable in practice. Until recently, these types of equipment have been cooled by such simple devices as fans and non-mechanical heat spreaders. The need for increased performance of such devices, together with ever increasing compactness, has led to greatly increased levels of heat dissipation from these devices, with the consequence that the conventional forms of cooling are in many cases unable to prevent device temperatures from rising too high, causing the devices to fail. Furthermore, the goat of some device designers is not merely to prevent harmful temperature rises but to facilitate performance-enhancing temperature decreases. For example, electronic equipment can run faster and can be more reliable if cooled sufficiently. Thus, a need exists not only for small-scale equipment that can cool devices to safe operating temperatures, but also to refrigerate devices further to temperatures that enhance performance.
Much effort has been devoted to improving the cooling of electronic components with forced air. Because space and cost considerations limit the size of fans that can be employed, greater attention is devoted to the heat sink that withdraws heat from a hot component by conduction, whereupon a fan cools it by forced convection. Lee (U.S. Pat. No. 5,653,285) provides a recent example of this, in which the heat sink is configured for maximum heat transfer efficiency. Another popular method of improving the heat sink is to construct it as a thermoelectric cooler, known as a Peltier cooler, which enables the temperature of the heat sink at the junction with the heat source to be substantially below the temperature of the heat source. Pettier coolers have the disadvantage of requiring more input power than can be dissipated and are therefore inefficient means of microrefrigeration.
There are many more such examples of heat-sink configuration. More recently, inventions have been revealed that employ a cooling fluid inside the heat sink. Miller et al. (U.S. Pat. No. 6,400,012) describe a heat sink with channels for the circulation of a coolant, the configuration of which is designed to reduce the thermal resistance between the heat generating device and the cooling medium within the heat sink. However, no specific cooling medium is provided. Cole et at. (U.S. Pat. No. 6,478,725) takes this concept a step further by providing a means by which a coolant comes in direct contact with the heat generating device, within a seated container, as an atomized spray, which then condenses and is drawn out of the container for circulation to another device for cooling, before returning to the sealed chamber to repeat the cycle. Such close proximity between coolant and microchip could in the long run disable the microchip.
More commonly, attention has been focused on indirect cooling in a small-scale heat exchanger of the type developed by Miller et al. Vafai et al. (U.S. Pat. No. 6,457,515) describe a two-layer microchannel structure. The cooling fluid is circulated through remote heat exchangers and other equipment in a distributed manner. In such distributed systems, the components of the system are separate and not enclosed in the same container. Konstad (U.S. Pat. No. 6,407,916) describe a more compact means of distributed heat removal, calling for a heat pipe to conduct a coolant back and forth between a heat sink and an air-cooled heat exchanger.
Refrigeration, as opposed to cooling, increases the complexity of design, especially if enclosing all components of the refrigeration cycle in a single container. Davidson et al. (U.S. Pat. No. 6,497,110) demonstrate this in a device that completely isolates the electronic components from surrounding devices, in part to prevent moisture condensation on said surrounding devices. A disadvantage is that wire connections for signal transmission to and from the electronic components are discouraged and are instead substituted by optical connections in the preferred version of that invention.
Current systems for refrigerating electronic equipment are bulky and can add to the overall size of the electronic appliance. Examples of such systems are disclosed by Porter (U.S. Pat. No. 5,574,627), Wall et al. (U.S. Pat. No. 6,054,676), and Eriksen et al. (WO 0125881 A2). The challenge in today's environment is to miniaturize such refrigeration systems so as to fit into existing appliance architecture, including laptop computers.
For reasons of compactness and efficacy, the working fluid employed in microrefrigeration devices with internal working fluid circulation, must be efficient in a thermodynamic sense. A thermodynamically efficient working fluid transfers heat readily with minimum requirement for work. For reasons of safety, the working fluid must also be nontoxic and environmentally benign. Performance demands such as this, lead us to consider transcritical carbon dioxide as the working fluid.
In conventional vapor-compression refrigeration cycles, heat is absorbed at constant temperature by a fluid undergoing evaporation, vapor is then compressed to a higher pressure before giving up heat of evaporation, as well as work energy added during compression, in a condenser at subcritical pressure, before ultimately decompressing through an expander and returning to the evaporator to pick up heat and begin the cycle anew. An alternative to this cycle is to compress the fluid to a supercritical state at a high enough pressure to ensure that it remains in a supercritical state as it releases heat to a cooling medium. In refrigeration cycles, the cooling medium is usually air, but it can be another fluid, such as seawater.
Then, as the cooled working fluid is expanded, it returns to a subcritical state and condenses, after which it returns to the evaporator to absorb heat anew. Such a cycle is termed transcritical.
Throughout the history of vapor-compression refrigeration, subcritical cycles have been the norm. Chlorofluorocarbon (CFC) working fluids operating on such cycles became popular in the 1930s. These fluids were deemed non-toxic and safe. By the early 1970s, however, the environmental risks posed by CFCs, particularly to the atmosphere's ozone layer, were realized. This has led to renewed interest in carbon dioxide, which can be operated in fully subcritical cycles, or in a transcritical cycle for better efficiency.